SensorDrop: A system to remotely detach individual sensors from wildlife tracking collars

Abstract The growing diversity of animal‐borne sensor types is revolutionizing our understanding of wildlife biology. For example, researcher‐developed sensors, such as audio and video loggers, are being increasingly attached to wildlife tracking collars to provide insights into a range of topics from species interactions to physiology. However, such devices are often prohibitively power‐intensive, relative to conventional wildlife collar sensors, and their retrieval without compromising long‐term data collection and animal welfare remains a challenge. We present an open‐source system (SensorDrop) for remotely detaching individual sensors from wildlife collars. SensorDrop facilitates the retrieval of power‐intensive sensors while leaving non‐resource‐intensive sensors intact on animals. SensorDrop systems can be made using commercially available components and are a fraction of the cost of other timed drop‐off devices that detach full wildlife tracking collars. From 2021 to 2022, eight SensorDrop units were successfully deployed on free‐ranging African wild dog packs in the Okavango Delta as part of audio‐accelerometer sensor bundles attached to wildlife collars. All SensorDrop units detached after 2–3 weeks and facilitated the collection of audio and accelerometer data while leaving wildlife GPS collars intact to continue collecting locational data (>1 year), critical for long‐term conservation population monitoring in the region. SensorDrop offers a low‐cost method to remotely detach and retrieve individual sensors from wildlife collars. By selectively detaching battery‐depleted sensors, SensorDrop maximizes the amount of data collected per wildlife collar deployment and mitigates ethical concerns on animal rehandling. SensorDrop adds to the growing body of open‐source animal‐borne technologies being utilized by wildlife researchers to innovate and expand upon data collection practices and supports the continued ethical use of novel technologies within wildlife studies.

mercial manufacturers. In response, researchers are increasingly adapting or developing new technologies for use in animal-borne studies, which has increased the diversity of sensor types available for wildlife research and enhanced the field's research capacity (e.g., Hernandez et al., 2018;McGregor et al., 2015;. Two common barriers to the uptake of novel animal-borne technologies are power requirements (with higher requirements typically necessitating shorter sensor deployment durations) and unit retrieval. Fundamental to many studies utilizing wildlife collars are the spatial data they provide, with additional auxiliary sensors commonly used to provide further context to animal movements (Kays et al., 2015). However, unlike GPS tags, for example, which are often both relatively power-un-intensive, due to the type of data they collect, and optimized for power efficiency, due to how long such sensors have been used in wildlife studies, new sensor technologies can have relatively high-power requirements (De La Rosa, 2019;Wijers et al., 2018). In some cases, for example, new wildlife sensors are adapted from technologies developed for industries where power efficiency is less of a consideration (such as sports action cameras, e.g., McGregor et al., 2015). Furthermore, even in cases where sensors are specifically designed for wildlife studies, the characteristics of the data they collect may necessitate relatively high-power requirements (such as audio data, e.g., Wijers et al., 2018).
As a consequence, when attaching new sensor technologies to wildlife collars, researchers must frequently navigate power limitations by either (i) recovering battery-depleted sensors shortly after deployment through animal rehandling or remote collar detachment or (ii) leaving battery-depleted sensors on wildlife until all collar sensors have finished collecting data. In the former, researchers have quicker access to data, and animals carry the additional weight of battery-depleted sensors for less time. However, there are ethical and welfare implications of (i) rehandling animals, which often induces stress and can involve the use of immobilization drugs that carry inherent risks (Kreeger & Arnemo, 2018;Meyer et al., 2008;Soulsbury et al., 2020), and (ii) deploying sensors for such short deployment durations, particularly in environments where multiple collaborating research partners have different data needs (e.g., long-term monitoring of populations versus short-term research projects). In contrast, by leaving depleted sensors on wildlife for longer, researchers can maximize the overall data collected per deployment effort, but animals must carry the additional weight of non-functional sensors, and researchers may need to wait multiple additional months to retrieve the data. Currently, no openly available system exists that allows supplemental sensors to be remotely detached from wildlife collars, with the best alternative involving adapting full collar dropoff mechanisms (e.g., from Vectronic Aerospace, Telonics, or Lotek Wireless) in ways technically challenging for the skillsets of many ecologists (see: wildl abs.net/discu ssion/ drop-pods-collars). ware design (presented below) to detach specific sensor bundles from wildlife collars and represents >800 h of additional development and field-testing time. SensorDrop is simple to use, is compatible with a wide range of sensor types, and is a fraction of the cost of comparable commercial drop-off mechanisms that detach full collars. Here, we provide a detailed system components summary and a case study of its use on free-ranging African carnivores to demonstrate the utility of SensorDrop in real-world settings.

| SYS TEM OVERVIE W AND COMP ONENTRY
SensorDrop is composed of four key components that together attach to a sensor housing of the user's design: (i) an OpenDrop printed circuit board (PCB); (ii) a drop-off plate; (iii) nylon line; and (iv) nylon webbing ( Figure 1). The total component cost of SensorDrop ranges from $25.36 to $60.77 per device, with costs decreasing as more units are needed due to decreasing part costs (Table S1).
The OpenDrop PCB is composed of a timer circuit that, at a userdetermined time, allows electrical current to flow to a nichrome element for a short pulse (~5 s), causing the nichrome element to heat and melt the nylon line securing the drop-off mechanism to the collar (see Rafiq et al., 2019). Within SensorDrop, the OpenDrop PCB is powered by a Tadiran 220 mAH 20C lithium-polymer battery (expected deployment duration of approximately 3 months) and connected to a coiled nichrome element, which heats to temperatures exceeding 150°C. Several factors interact to control the heat of the nichrome element, mainly the lengths of the connecting wires and the gauge and format (i.e., coiled or straight) of the nichrome element. Coiling the nichrome, for example, has the effect of intensifying the heat generated as current runs through the wire.
Moreover, as you increase the length of any of the connecting wires, the relative resistance within the circuit increases and the maximum temperature the nichrome element reaches decreases. In contrast, decreasing the length of the connecting wires will achieve the opposite and can, if the nichrome element contacts other components of the sensor housing, lead to open flames. As such, we refer users to the configurations provided within the SensorDrop online repository as an initial starting point, and we encourage thorough testing in safe and controlled environments before any field deployments.
Additionally, for safety, nichrome elements should remain fully contained within sensor housings to avoid contact with the natural en-

vironment (animals and vegetation). Full details on programming the
OpenDrop PCB can be found in Rafiq et al. (2019) and the associated online repository.
The SensorDrop drop-off plate connects the main sensor housing that the user wishes to detach, the nylon webbing, and the OpenDrop PCB ( Figure S1). Files for 3D printing, CNC-milling, and modifying previously used sensor housings compatible with SensorDrop can be found within the SensorDrop online repository. Depending on species and location-specific stressors expected to be placed on the wildlife collars, the drop-off plate can be 3D printed and coated with finishing resin for water resistance (e.g., X3D Finishing Epoxy Resin) or CNCmilled for greater strength. The plate contains two parallel rows of five holes at opposing ends, with a polycarbonate strip (2 mm thickness) attached across one set of holes. The nichrome element is suspended across the polycarbonate strip using brass spacers in order to avoid To increase the unit's water resistance, silicon adhesive can be used to fill the plate holes prior to the nylon line being looped through. The appropriate number of nylon lines to use and their tensile strength will depend on the specifics of the study system, particularly the forces expected to be placed on sensors during deployment. Too few or weak lines will result in premature detachment of the unit, for example, with contact from the environment severing lines. In contrast, excessive or overly strong lines can increase the mass of units as larger batteries with higher current capabilities are usually required to sever nylon lines. For example, for social species where sensors can be expected to be pulled, twisted, or bitten by conspecifics, or for species occurring within densely vegetated woodlands where sensors can knock against vegetation, higher numbers of, or higher tensile strength, nylon lines, may be preferred. We recommend increasing the number of nylon lines used to secure the drop-off plate to the nylon webbing before increasing the tensile strength as thicker lines may require significantly higher current in order for the nichrome element to reach the temperatures able to sever the line. As a general starting point, we recommend using the number and tensile strength of lines that, in sum, match or exceed twice the weight of the mean mass of individuals to be collared Once the SensorDrop drop-off plate and nylon webbing are secured together via the nylon line, the drop-off plate is screwed onto the user-designed sensor housing and the nylon webbing is bolted or riveted onto the wildlife collar. At the user's pre-programmed time the nichrome element will heat, melting the nylon line and severing the sensor bundle from the nylon webbing, thereby allowing users to collect detached sensors. Additionally, a VHF transmitter can be embedded into sensor bundles to assist with finding detached units (see case study below), which we would recommend for most use cases given the challenges of finding devices among vegetation. However, for deployments within controlled systems (e.g., captive animals) or species with predictable ranging patterns during deployment (e.g., denning mammals), VHF transmitters may not be required. Upon detachment, the only SensorDrop components left on the wildlife collar are the nylon webbing and nylon lines (weighing ~4 g), thereby minimizing wildlife collar weight from non-essential supplemental material.

| C A S E S TUDY
We configured all SensorDrop units to remotely detach from wildlife collars 2-3 weeks after sensor deployments, and we visited collared individuals every 2-4 days to collect supplementary data and assess welfare. No ill effects of collar deployments were observed. All SensorDrop integrated units detached within 2-3 weeks following deployments and facilitated the collection of audio and accelerometer data that were available for immediate use, while leaving wildlife GPS collars intact to continue collecting locational data (>1 year), critical for long-term conservation population monitoring in the region. Seven of the units detached at the pre-programmed times. One unit detached prematurely after water ingress led to the drop-off mechanism triggering 6 days early. This was a fail-safe mechanism implemented within the OpenDropOff software to ensure units dropped if SensorDrop battery voltages reached user-defined critical thresholds, for example, during short circuits caused by water immersion, as in our case (Rafiq et al., 2019). Further investigation of the unit indicated that the water ingress was likely due to the collared individual spending more than anticipated time crossing flooded areas within our study site.

| DISCUSS ION
SensorDrop offers a low-cost method to remotely detach and retrieve individual sensors from wildlife collars. SensorDrop allows researchers to remotely retrieve power and data-intensive sensors while leaving longer-lasting sensors intact on animals. This has While more affordable bio-degradable drop-off mechanisms exist, which typically function by detaching wildlife collars once a biodegradable link in the collar decomposes, detachment times can be highly variable and unpredictable, with collars often separating several months before or after when expected (Hellgren, 1988